The recent development of silicon (Si) substrates with strained layers has increased the options available for design and fabrication of field-effect transistors (FETs). Enhanced performance of n-type metal-oxide-semiconductor (NMOS) transistors has been demonstrated with heterojunction metal-oxide-semiconductor field effect transistors (MOSFETs) built on substrates having strained silicon and relaxed silicon-germanium (SiGe) layers. Tensilely-strained silicon greatly enhances electron mobilities. NMOS devices with strained silicon surface channels, therefore, have improved performance with higher switching speeds. Hole mobilities are enhanced in tensilely-strained silicon as well, but to a lesser extent for strain levels less than approximately 1.5%. Equivalent enhancement of p-type metal-oxide-semiconductor (PMOS) device performance, therefore, in such surface-channel devices presents a challenge.
A structure that incorporates a compressively strained SiGe layer in tandem with a tensilely strained Si layer can provide greatly enhanced electron and hole mobilities. In this structure, electron transport typically occurs within a surface tensilely strained Si channel and hole transport occurs within the compressively strained SiGe layer below the Si layer. To support the fabrication of NMOS transistors as well as PMOS transistors on this structure, the surface tensilely strained Si layer has a typical thickness of 50–200 Ångstroms (Å) for providing a channel for conduction of electrons. If this layer is thinner than 50 Å, the beneficial mobility enhancement is significantly reduced because the electrons are no longer completely confined within the strained Si layer. Although some NMOS devices are operational with a strained silicon surface channel of only 50 Å, even this strained silicon layer thickness may be too thick to allow modulation of p-type carriers in a buried SiGe layer by an operating voltage applied to the gate of a PMOS transistor.
Complementary metal-oxide silicon (CMOS) circuit design is simplified if carrier mobilities are enhanced equally for both NMOS and PMOS devices. In conventional silicon-based devices, electron mobilities are approximately two times greater than hole mobilities. As noted, electron mobilities have been substantially increased with strained silicon. Methods for equally increasing hole and electron mobilities by forming dual-channel NMOS and PMOS devices on the same substrate are problematic, in part because of different surface strained-silicon thickness requirements for the two types of devices.